After playing Neverwinter Nights for a few months, I got tired of it. I played the main campaigns and enjoyed them. The problem is that the mods are much harder to deal with than they should be. It takes a lot of effort to find mods that you can play (level restrictions, number of players, good ratings, etc.). It then takes more effort than it should to download and install them. They come in ZIP, EXE, and RAR formats. They come with multiple files, each of which may have to be put into a different folder. Bioware could have made mods easier by keeping all the mod files in a single folder and by standardizing on the distribution format. For example, if the game could read ZIP files, you could just download the file and put it in a particular folder and it would be ready, without unpacking it. In any case, the main problem I had was that it was hard for me to find and try out mods, so I didn't try many, and I didn't find many that I liked.
I've been playing Guild Wars this week (22 hours so far). This is an action RPG, much like Diablo, but it also has more quests. One thing that struck me was that Diablo 2 wasn't considered an MMORPG, but Guild Wars sometimes is, and the two games aren't substantially different in that respect.
Diablo 2 lets you play offline (one player) or online (one or multiple players). Guild Wars lets you play online only. This seems like a good idea. If you know that everyone's online, you can have a much quicker update cycle (those of you who waited over a year for the 1.09 Diablo 2 patch know why this matters). So far I've had 3 small updates in 4 days. Requiring online play also means you can learn how people play and make game balance adjustments frequently.
Diablo 2, in online mode, puts you in one of many chat rooms. The main purposes of this room are to chat, to find people to play with, and to trade. There are several chat rooms (presumably to avoid overcrowding), including some specifically for trading. When joining a game, you see a list of games that are at your level (Normal/Nightmare/Hell). People generally want to find a game that's not only at their level but in the same general areas. They sometimes do this by announcing their area (“Act III - Staff quest”) in the game name, but there's no easy way to filter or sort the games to find the areas you care about. Once you join a game, you don't see everyone anymore; you only see your party. Every time you join a game, the world resets—all the monsters are back, and the layout is randomized. When you leave a game, you abandon your party.
Guild Wars feels like an MMORPG because it puts you into the game right away. You're in a city, with lots of other people around, and you can see everyone walking around, talking, getting quests, etc. But these cities are essentially chat rooms. Their main purpose is for people to chat, find people to play with, and trade. Once you join a party and go out to play the game, you don't see everyone anymore; you only see your party. Every time you go out, the world resets—all the monsters are back (but the layout is not randomzied as it was in Diablo). When teleport into a city, you abandon your party.
There are of course other differences. In Diablo, you can only get quests once you go into the game. In Guild Wars, you can get quests in the chat area (cities). Since a party may want to go into a city to get quests, the party is not broken up if you walk into a city; it's only broken up if you teleport. In Diablo, if you want to trade with someone, you find them in the chat room and then you have to create a game to trade with them. In Guild Wars, if you want to trade with someone, you can trade with them in the city where you met them; there's no need to go anywhere. Also, Guild Wars has a special trading chat channel. You can tune in to both the regular and the trading channel; you don't have to go somewhere to find the traders. Guild Wars also makes it easier to find a party or to play PvP.
Why does Guild Wars keep the Diablo-style chat vs. game area distinction? I think it's for efficiency. Chat areas handle hundreds of people; game areas handle up to four. Chat areas are easier to make efficient because you don't need strict ordering of events. If you say something and I say something at roughly the same time, it doesn't matter so much which is heard first. And it doesn't matter so much if it takes 250 ms to show up. If you attack someone and I attack someone at the same time, it matters a great deal which one occurred first, and it needs to be decided and sent to all clients much quicker than 250 ms. By segregating the chat (lots of clients, loose ordering of events, high latency) and the game (few clients, strict ordering of events, low latency), you can make both efficient. If you tried to combine the two, you open up a new hard problem: what happens when you have lots of clients and you need low latency? Trying to tackle that rare but important case makes your life as a game developer more difficult. You're spending less time on other things that may have mattered more to the players.
Another thing that Guild Wars doesn't handle is massive persistence. You can only drop items in the game areas, not the chat areas. So if you drop an item, it only has to be seen by a few other clients, and it goes away as soon as you enter a city. That way you don't need to coordinate item locations among tens of thousands of clients. You can assign each city area and each game area to a separate server. Since city and game servers keep no state, you can add more servers transparently. You don't have to require players to choose a server ahead of time and then find that they can't play with their friends, who happen to live on a different server.
So far I've been really impressed with what I can infer of the design of Guild Wars. It's fast, has great graphics, and lots of fun. It's an incredibly addictive game.